1. Field of the Invention
The present invention relates to a brushless motor, in particular, to improvements of a sealing structure of a bearing portion in a spindle motor used for HDD (hard disc drive) and other disc type driving device.
2. Description of the Related Art
Presently, in order to rotate a storage disc such as "Hard Disc" (referred to as "HD") in a sealed housing of disc storage systems, a Hard Disc Drive (HDD) is widely used.
As a spindle motor for driving the HDD in a prior art, a brushless motor for instance, is employed as shown in FIG. 1.
FIG. 1 is a vertical sectional view showing a right half side of a brushless motor along a center axis of a shaft in a prior art.
Referring to FIG. 1, a hub 16 is rotatably provided on a shaft 10 through bearings 12, 14. The shaft 10 is integrally formed on a motor base 18 which is fixed on a chassis (not shown). A plurality of coils 20 each wound around a core made of a magnetic material are provided around a bottom portion of the shaft 10 in a lower position, and a magnetic 22 having a plurality of magnetic poles is provided on an inner surface of a yoke formed in the hub 16 in such a manner that the plurality of magnetic poles of the magnet 22 respectively face the plurality of coils 20.
The hub 16 is provided with a disc holder 24 on an outer surface thereof in a lower position on which a lower hard disc 28 is clamped by a clamper 26. Further, another upper hard disc 28 is clamped on the clamper 26 by another clamper 30 which is fixed on a upper surface of the hub 16. An access to each of the upper and lower hard discs 28, 28 is made from both the sides thereof by a pair of magnetic heads 32a, 32b.
Further, a sleeve 34 is provided on the shaft 10 in a upper position, and a ring magnet 36 is provided on a inner surface of the hub 16 in such a manner that the ring magnet 36 faces to the sleeve 34. Magnetic fluid seal 38 is filed between the sleeve 34 and the ring magnet 36 for sealing the bearings 12, 14, so that contaminant particles scattered from the bearings 12, 14 and the coils 20 are prevented from invading an area EB (referred to as clean room) sealed from the outer environment EA, in which clean chamber EB the hard discs 28, 28 are installed.
Upon operation, the coils 20 is applied with a current from a switching circuit (not shown) by which a direction and an interval of the current are controlled so that the hub 16 is rotated in a direction of an arrow FA due to an interaction between a magnetic field produced by the magnet 22 and a magnetic field generated by the coils 20. Thus, the upper and lower hard discs 28, 28 are rotated with respect to the pairs of magnetic heads 32a, 32b. The pairs of magnetic heads 32a, 32b give access to desired tracks of the upper and lower hard discs 28, 28 by being traversed in right and left directions.
In the above prior art, however, there are disadvantages as follows:
(1) Generally in a brushless motor like shown in FIG. 1, the bearings 12, 14 and coils 20 scatter contaminant particles such as dust and grease particles, which invade the clean chamber EB to contaminate the atmosphere therein, thus, the Contact Start and Stop characteristic (CSS) of the HDD is degraded even when the contaminant particle sizes are as small as 0.1.about.0.3 .mu.m. In the aforementioned prior art, the magnetic fluid seal 38 is employed to prevent the contaminant particles from invading the clean chamber EB where the hard discs 28, 28 are installed, which poses a cost increase of the brushless motor because of a high price of the magnetic fluid seal 38. PA1 (2) As shown in FIG. 1, however, the contaminant particles of dust and grease have chances to invade the clean chamber EB through gaps between the magnet 22 and the coils 20, these also degrades cleanness of the atmosphere of the clean chamber EB. PA1 (3) Recently, there is a demand for down-sizing of the HDD so as to enable an employment of the HDD in a notebook-type personal computer, so that its spindle motor is also required to have a good sealing characteristic against the contaminant particles without increasing a size and weight thereof.
In order to decrease the size and weight of a brushless motor, it is conceivable for the brushless motor to employ coreless coils instead of the coils 20 having cores made of magnetic materials.
A description is now given to an assembly method of the coreless coil in a prior art.
FIG. 2(A) is a perspective view of a plane coil of a prior art before the plane coil is assembled to a motor.
FIG. 2(B) is a fragmentary view taken in the direction of the arrows along the #2--#2 line of FIG. 2(A). FIG. 2(C) is a plan view showing a metal mold for preforming plural pieces of the plane coils shown in FIG. 2(A). FIG. 2(D) is a perspective view showing a configuration of the plane coil after it is preformed.
Referring to FIGS. 2(A) and 2(B), at first, a plane coil 90 is formed so as to have a flat configuration by winding a wire 92 around a frame or without the frame.
Next, referring to FIG. 2(C), a predetermined plurality of the plane coils 90 are disposed in a space defined by a metal mold jig 94 having a cylindrical configuration and a pair of metal mold jigs 96, 96 each having an arcuate cylindrical recess. Then, the plurality of the plane coils 90 are pushed against the metal mold jig 94 by causing the pair of metal mold jigs 96, 96 to displace in directions of arrows FA, FA, so that the plurality of the plane coils 90 are bent along a periphery of the metal mold jig 94. Thus, each of the plurality of plane coils 90 forms a arcuate cylindrical shape corresponding to the cylindrical configuration of the metal mold jig 94 as shown with a chain line in FIG. 2(D).
Next, after resin is filled into the space formed by the metal mold jigs 94, 96 in a direction of an arrow FB, the resin is cured by heat, thus, a coreless coil assembly 98 is obtained as shown in FIG. 3.
FIG. 3 is a perspective view showing a coreless coil assembly of a prior art.
It is conceivable that the coreless coil assembly 98 is provided, for instance, on a stator side in the brushless motor so that the hub 16 is rotated by an interaction between the magnetic field produced by the magnet 22 and the magnetic field generated by the coreless coil assembly 98.
In the above prior art, however, the contaminant particles such as dust and grease particles can not be prevented from invading the clean chamber EB through gaps between the magnet 22 and the coreless coils 90, which degrades the cleanliness of atmosphere of the clean chamber EB.